The present invention relates to a seat belt retractor for retracting a seat belt, i.e., the seat belt retractor is configured to enable a seat belt to be withdrawn from it in certain circumstances and is configured to retract the seat belt in other circumstances. More specifically, the present invention relates to a seat belt retractor including an energy absorption (“EA”) mechanism for absorbing and alleviating impact energy of the occupant. Conventionally, the energy is absorbed by limiting a load that is exerted on the seat belt by means of a torsional deformation of a torsion bar; the deformation of the torsion bar prevents the seat belt from being withdrawn in an emergency such during a collision.
During the aforementioned emergency, large vehicle decelerations occur, and accordingly, the occupant is moved forward due to inertia. Consequently, a heavy load is exerted on the seat belt. As a result, the occupant receives a large impact force from the seat belt. Though this impact force is not a particular problem for the occupant, limiting this impact force is preferable for the occupant.
Seat belt systems (including seat belt retractors) provided in vehicles such as automobiles prevent occupants from flying from a seat and, therefore, protect the occupants by restraining them in the aforementioned emergency. Seat belt retractors retract a seat belt onto a spool when an occupant does not wear the seat belt. By way of contrast, when the seat belt is worn, the seat belt is withdrawn from the spool. As part of these seat belt retractors, a locking mechanism acts in the aforementioned emergency and prevents rotation in a seat belt-withdrawing direction of the spool, thereby preventing the seat belt from being withdrawn. Thus, the seat belt can firmly retain and protect the occupant.
To this end, some conventional seat belt retractors (such as that disclosed in Japanese Unexamined Patent Application Publication No. 2001-58559) have been developed with an arrangement wherein the load that is exerted on the seat belt is limited by providing a torsion bar. As a result of the torsion bar, impact energy is absorbed and alleviated while the occupant wears a seat belt in an emergency.
FIG. 5 is a longitudinal sectional view illustrating an embodiment of the seat belt retractor disclosed in Japanese Unexamined Patent Application Publication No. 2001-58559. In the drawing: (a) 1 denotes a seat belt retractor; (b) 2 denotes a frame in the shape of a square with one end open; (c) 3 denotes a seat belt; (d) 4 denotes a spool rotatably supported between both side walls of the frame 2 in the shape of a square with one end open to retract the seat belt 3; (e) 5 denotes deceleration sensing means for detecting large vehicle decelerations such as during the aforementioned emergency; (f) 6 denotes a locking mechanism that is activated by the deceleration sensing means 5 to prevent the spool 4 from rotating in the belt-withdrawing direction; (g) 7 denotes a torsion bar for loosely fitting, penetrating, and rotationally interlocking the spool 4 and locking mechanism 6 in an axial direction centered on the spool 4; (h) 8 denotes spring means for constantly pressing the spool 4 in a belt-retracting direction via a bush 10 with the spring force of a spiral spring 9; (i) 11 denotes a pretensioner for acting in the aforementioned emergency so as to generate a belt retracting torque; 0) 12 denotes a bush for transmitting the seat belt retracting torque of the pretensioner 11 to the spool 4; (k) 15 denotes a male screw shank 15 that is formed on the locking base 14; and (1) 16 denotes a nut-shaped stopper member 16 screwed onto the male screw shank 15 for rotating integrally with the spool 4.
The locking mechanism 6 comprises a locking base 14 and a lock gear 6a. The locking base 14 is capable of rotating integrally with a later-described first torque transmitting member 17 of the torsion bar 7, for supporting a pawl 13 so as to be oscillated. The lock gear 6a, which normally rotates integrally with the torsion bar 7, in an emergency stops rotation by operation of the deceleration sensing means 5. During such an emergency, the lock gear 6a generates a relative rotational difference that causes the pawl 13 to engage with an internal gear 19 of the side wall of the frame 2. As a result, rotation of the locking base 14 is prevented, i.e., the spool 4 can not rotate in the belt-withdrawing direction.
The torsion bar 7 is formed with: (a) a first torque transmitting member 17 for engaging with the locking base 14 so as not to perform relative rotation; and (b) a second torque transmitting member 18 for engaging with the spool 4 so as not to perform relative rotation.
The spool 4 is constantly pressed in the belt-retracting direction by the spring force of the spring means 8 via the bush 10, torsion bar 7, the second torque transmitting member 18 of the torsion bar 7, and bush 12. Also, the belt retracting torque generated at the pretensioner 11 is transmitted to the spool 4 via the bush 12 at the time of operation of the pretensioner 11 and, therefore, the spool 4 retracts the seat belt 3 by a predetermined length.
With the conventional seat belt retractor 1 thus configured, the seat belt 3 is completely retracted by the pressing force of the spring means 8 when the occupant does not wear the seat belt 3. When the occupant withdraws the seat belt 3 to wear it, the spool 4 rotates in the belt-withdrawing direction, and the seat belt 3 is smoothly withdrawn. A buckle tongue (not shown in FIG. 5), which is slidably provided on the seat belt 3, is inserted in a buckle latch (not shown in FIG. 5) fixed to the vehicle so as to be latched. Finally, the seat belt 3 is partially retracted in the belt-retracting direction so as to fit snugly against the occupant while not constricting the occupant.
During an emergency, the seat belt retracting torque generated at the pretensioner 11 is transmitted to the spool 4, the spool 4 retracts the seat belt 3 by a predetermined length, and restrains the occupant firmly. On the other hand, the locking mechanism 6 is activated by operation of the deceleration sensing means 5 due to the large vehicle deceleration that occurs in an emergency. In other words, rotation of the locking gear 6a in the belt-withdrawing direction is prevented by operation of the deceleration sensing means 5 by forcing the pawl 13 of the locking mechanism 6 to rotate and engage with the internal gear 19 on the side wall of the frame 2. Thus, rotation of the locking base 14 in the belt-withdrawing direction is prevented. Accordingly, the torsion bar 7 is subjected to torsion and only the spool 4 rotates relative to the locking base 14 in the belt-withdrawing direction. Thereafter, the spool 4 rotates in the belt-withdrawing direction while subjecting the torsion bar 7 to torsion. The torsion torque of the torsion bar 7 limits load to be exerted on the seat belt 3, and consequently, the impact that would otherwise be exerted on the occupant is absorbed and alleviated.
The stopper member 16, which rotates integrally with the spool 4, rotates relative to the male screw shank 15 and, accordingly, moves toward the locking base 14. Subsequently, when the stopper member 16 contacts the locking base 14, further rotation of the stopper member 16 is prevented and, therefore, further rotation of the spool 4 is also prevented and torsion of the torsion bar 7 is stopped. Thus, the seat belt 3 is prevented from withdrawing, the occupant is firmly restrained by the seat belt 3, and the maximum torsion of the torsion bar 7 is limited, thereby preventing the torsion bar 7 from being sheared as a result of the torsion applied thereto.
This conventional seat belt retractor 1 is configured such that the locking base 14 of the locking mechanism 6 rotates relative to the locking gear 6a in the belt-withdrawing direction even in the event that the seat belt is rapidly withdrawn. Thus, when the pawl 13 of the locking mechanism 6 is engaged with the internal gear 19 on the side wall of the frame 2 as previously described, rotation of the locking base 14 is prevented, rotation of the spool 4 in the belt-withdrawing direction is prevented via the torsion bar 7, and consequently, the seat belt 3 is prevented from withdrawing.
Furthermore, in Japanese Unexamined Patent Application Publication No. 2001-58559, a seat belt retractor 1 is disclosed wherein the load limit (FL load) can be switched. More specifically, as shown in FIG. 6, in the initial stage following a collision, a locking member 38 pops out from a cylinder member 37 due to the spring force of a spring 39 acting upon a lock pin 40 that is blown away by gas discharged from the pretensioner 11 (not shown in FIG. 6). Thus, a ratchet pawl 35a of a lock wheel 35 is engaged with the locking member 38, and the lock wheel 35 is prevented from rotating in a belt-withdrawing direction. Consequently, a second torsion bar 32 is subjected to torsion at a portion of length 11 shorter than the total length θ. Thus, EA operation due to the EA mechanism is started by means of torsion of the first torsion bar 7 and torsion of the second torsion bar 32; the FL load F1 at this time is relatively large, as shown in FIG. 7.
The second torsion bar 32 is subjected to fracture due to a predetermined amount of torsion. As a result, after the fracture of the second torsion bar 32, the EA operation is performed only by the first torsion bar 7; the FL load F2 at this time is relatively small compared to the FL load F1 at the initial stage, as shown in FIG. 7. Thus, the FL load is switched and, therefore, the load limit to be exerted on the seat belt can be set flexibly and widely, depending on the restraining state of the occupant in an emergency by an airbag, and the like.
However, while the seat belt retractor 1 disclosed in Japanese Unexamined Patent Application Publication No. 2001-58559 can set the load limit widely depending on the restrained status of the occupant, the load limit at the initial stage following a collision is set with only one kind of load limit. With the initial stage following a collision, impact energy to be exerted on the occupant is large. However, this impact energy varies widely depending on the weight of the occupant, the slide position of the seat, the collision speed, the manner of the collision, and the like. Thus, it is difficult to restrain the occupant more effectively and appropriately by handling large impact energy that varies widely at the initial stage with only one kind of load limit.
Accordingly, the present invention has been made in light of the aforementioned problems. It is an object of the present invention to provide a seat belt retractor including load limit setting means that is capable of flexibly and widely setting the load limit to be exerted on a seat belt depending on the situation in an emergency. Moreover, it is a further objection of the present invention to provide a seat belt retractor in which the aforementioned load limit setting means is reusable.